State-of-polarization control systems have applications with fiber sensors, interferometry and optical communications systems. The following background information is presented herein only by way of example with reference to a polarization controller for use in an optical fiber communication system.
One type of conventional optical fiber communication system employs a single-mode fiber that transmits a light beam emitted by a narrow spectral linewidth semiconductor laser. An optical modulator modulates the light beam in accordance with an electrical message signal at baseband frequencies to form a modulated optical signal that propagates through the single-mode fiber. The polarization state of the modulated optical signal typically changes over time at the communication system receiver as a result of thermal or mechanical disturbances or stresses undergone by the fiber or as a result of inherent birefringence of the fiber. For example, a linearly polarized transmitted optical signal typically becomes generally elliptically polarized by the time it reaches the receiver. Such changes in polarization state require compensation to enable the use of a coherent detection receiver. The cost and drawbacks of using conventional polarization-maintaining fiber preclude its use in practical communication systems and plain coherent detection schemes. However, the polarization state of a signal in a single-mode fiber varies slowly enough to permit polarization compensation.
A state-of-polarization matching scheme may be implemented in a variety of ways and can be incorporated into existing fiber optic networks. Two basic approaches for controlling the polarization state of a signal are (1) matching the polarization state of a locally generated signal with that of the received communication signal and (2) separately detecting the two orthogonal polarization components of the received communication signal and adding them together after appropriate polarization compensation.
Combinations of these approaches generally employ two or more controlling elements to compensate for the number of degrees freedom of a polarization state, i.e., the ellipticity and tilt angle. The controlling elements previously employed in polarization controllers include electromagnetic fiber squeezers, electrooptic crystals, rotatable fiber coils, rotatable quarter-wave and half-wave plates, Faraday rotators, and rotatable fiber cranks.
Each of these polarization controllers suffers from varying degrees of insertion loss, mechanical fatigue, and other disadvantages, which are more fully described by Okoshi, "Polarization-State Control Schemes for Heterodyne or Homodyne Optical Fiber Communications," Journal of Lightwave Technology, IEEE Vol. LT-3, No. 6, December, 1985. Other disadvantages associated with the polarization controllers are their high cost and need for high operating voltages.
Two polarization controllers that are capable of providing endless control are rotatable wave plates and rotatable fiber cranks. Endless control is important because the fluctuation of the polarization state of a signal in a single-mode fiber is unpredictable. Thus, a state-of-polarization control element having a limited control range might require resetting. The resetting in conventional systems is generally accompanied by loss of the polarization state of the local optical signal and a consequent loss of information because of an inability to maintain continuous coherent detection. This loss can be substantial if the polarization state of the received optical signal is near to or fluctuates about the critical range points of the polarization controller elements.